A blind fastener of the type with which this invention is most concerned is shown in Wing and Schuster patent No. 3,129,630. This fastener has enjoyed continuous and very substantial sales to this day, and has been the subject of many improvements.
This type of fastener has a tubular sleeve with a shank, a tapered end, and a head at its other end. A threaded mandrel is threaded into and through the shank. The mandrel has a head that faces toward the tapered end of the sleeve. A cylindrical collar is placed on the mandrel, and turning the mandrel draws its head against the collar, forcing the collar onto and over the sleeve, thereby forming an enlargement. The combination of the head on the sleeve, and the enlarged collar, pulls and holds the workpieces together, and the shank provides resistance to shear forces. The device then functions as a blind rivet or as a blind bolt.
The mandrel and the head of the sleeve are provided with torque tool engaging mans so the mandrel can be turned in the sleeve to set the fastener. A groove is formed in the mandrel so that excess length of the mandrel will torque off at that location after the fastener has been set.
This fastener, commonly known by its trade names "Beta Bolt" and "Beta-Lok", has been sold successfully by Hi-Shear Corporation of Torrance, Calif. for many years, and still is. In order to make the device more useful and convenient for assembly operations, attention has been given to the accessible end of the fastener. The fastener parts which remain in the completed assembly after setting remain basically unchanged.
In the Wing and Schuster device, tool engaging recesses are formed in the head of the sleeve. The tool which sets the fastener has a nose with projections that enter into these recesses to hold the sleeve against rotation. The tool also includes a drive portion that engages the mandrel, and when turned rotates the mandrel so it moves axially to set the fastener.
Disadvantages of this arrangement are that the nose pieces wear out, that their sharp protruding end features which engage the recess can slip out and mar a surface, and that the portions of the mandrel which break off are relatively small, and sometimes tend to get away from the assemblers.
It has been suggested that, instead of engaging the head of the fastener, the tool could instead be provided with means to engage a nut threaded onto a lengthened mandrel adjacent to the head of the sleeve. Then the nut would be turned down against the head of the sleeve to jam on it. This would constitute means to hold the sleeve against rotation, and the nut would be a throwaway item that would keep the torqued off portion of the mandrel under control. An example of such a device is shown in U.S. Pat. Nos. 4,772,167 and 4,752,169.
This approach has some advantages, but often requires surface treatment to the fastener head, or discontinuities in it to assure enough retention forces to prevent relative rotation of the nut and the sleeve. It is an object of this invention to improve this situation and to make possible a truly effective use of a fastener of this type which does not require special head treatment, or tool engaging recesses.
There is yet another problem in devices wherein a mandrel stem torques off at a breakneck groove. When the end twists off it tends to leave behind a rough stem surface which may even include sharp pointed regions. These are not acceptable in high quality aircraft construction.
One of the embodiments of this invention provides the advantage that when the drive portion of its mandrel torques off, it leaves behind in the set fastener a smooth planar surface which is acceptable without further treatment.